1. Field of the Invention
This invention relates to a drill for boring holes in a rigid workpiece of a stone, concrete, metal, etc. More particularly, this invention relates to a high-speed rotary drill provided along its axis with a fluid passage through which a fluid such as air is supplied so as to blow away chips generated in drilling the rigid material.
2. Description of the Prior Art
As a typical drill for drilling rigid materials of a stone, concrete, metal or the like, there has been known an impact twist drill having a cutting bit with a striking edge. With this twist drill, drilling work to bore a hole into the rigid material is carried out by rotating the drill at about 700 to 1000 rpm under pressure and simultaneously imparting vibrations in the axial direction to the drill. Because the conventional drill of this type is subjected to rotation and vibration in the axial direction, the rotational speed of the drill could not be increased, and consequently, the efficiency of drilling could not be improved. Besides, vibrations and noises generated in drilling cause an environmental nuisance and exert a harmful influence upon not only the drilling machine but also the material to be drilled.
The inventor of the present invention has proposed drills capable of nonshock, high-speed drilling in Japanese Utility Model Application Public Disclosure SHO No. 60(1985)-87650(A) and Japanese Patent Application Public Disclousure SHO No. 61(1986)-146412(A). Every drill proposed by the inventor is provided on its leading end with a cylindrical cutting bit which is formed by sintering ultra-hard abrasive grains including diamond grains and can fully withstand the drilling work carried out at high speed. Furthermore, the proposed drill has a fluid passage formed along the axis thereof so as to introduce a fluid such as air therethrough and spout it from the cutting bit disposed at the leading end of the drill, thereby to effectively remove cutting chips generated in drilling.
However, though the prior art drill makes it possible to fulfill the effective drilling work at high speed, another problem in relationship between the drill and the drilling machine arises. This will be explained as follows.
In order to facilitate attachment and detachment of the drill and the drill holder of the drilling machine and attain compactness of the drilling system consisting of the drill and the drilling machine, the drilling system adopts a screwed type coupling structure constituted by a male thread formed on the drill and a female thread formed inside the drill holder. However, compared with a conventional twist drill which rotates at relatively low speed of less than 1000 rpm to drill a given object, the aforementioned high-speed drill has to be strongly coupled with the drill holder of the drilling machine. Generally, the screwed type coupling structure in the drilling system as mentioned above has a fine clearance between the male thread of the drill and the female thread of the drill holder of the drilling machine in order to perform the attachment and detachment operations of the drill and the drill holder with ease, whereas the clearance causes wobbling of the drill which rotates at high speed.
Another reason why the coupling strength between the drill and the drill holder of the high-speed drilling machine should be improved is that the drill in drilling the rigid material at high speed suffers enormous torque.
Also, the drilling system of this type which is applied for drilling the rigid material has called for a chip exhausting function of effectively removing cutting chips produced in drilling.